Various cofactors are known to increase the rate of HIV infection and to cause asymptomatic seropositive individuals to more rapidly develop AIDs. The role of common infectious diseases as cofactors in enhancing both virus transmission and disease progression is of particular interest in human medicine but is difficult to study. Humans cannot be purposefully infected, pathogen free populations do not exist, and people cannot be put into sterile environments once they are infected. This aspect of HIV infection can be better studied using an appropriate animal model. Feline Immunodeficiency Virus (FIV) infection might provide such a model. Specific pathogen free (SPF) cats are available, experimental infections can be created, and various infectious disease exposing environments can be artificially recreated. We propose to study the role of secondary infections as cofactors in FIV transmission and disease progression in two way: 1) by exposing FIV-infected and non-infected specific pathogen free cats to a wide range of common feline pathogens by natural means, i.e. by contact exposure with conventional cats housed in the same rooms, and 2) by sequentially infecting FIV-infected and non- infected specific pathogen free cats with 7 common feline pathogens, feline herpesivirus, feline calicivirus, feline enteric coronavirus, Chlamydia psittaci, hemobartonella felis, Toxoplasma gondii, and feline leukemia virus. The purpose of the first study is to see if continuous exposure to many different secondary disease agents enhances the transmission of FIV between infected and susceptible cats or accelerates the progression from the asymptomatic to the AIDS-phase of illness. The aim of the second study is to see whether FIV infection in asymptomatic FIV-infected cats alters the normal clinical and immunologic manifestation of 7 common infectious diseases of cats, alters the carrier state that follow the clinical phase of these illnesses, affects the normal perturbations in lymphocyte blastogenesis that appear transiently during and after most common infectious processes. Conversely, we are interested in seeing whether coinfection of FIV infected cats with any of these agents alters the levels of FIV in their blood or accelerates their immunologic decline.